A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, one or more alignment marks are generally provided on, for example, the substrate, and the lithographic apparatus includes one or more alignment sensors by which the position of the mark may be measured accurately. The alignment sensor may be effectively a position measuring apparatus. Different types of marks and different types of alignment sensors are known from different times and different manufacturers.
Known alignment sensors use one or several radiation sources to generate a plurality of radiation beams with different wavelengths. In this fashion, a sensor may measure position using several wavelengths (e.g., colors) and polarizations of radiation (e.g., light) on the same target grating or gratings. No single color or polarization is ideal for measuring in all situations, so the system selects from a number of signals, which one provides the most reliable position information.
As substrates become increasingly complex, with increasing numbers of patterns being applied to them, it becomes necessary to add additional wavelengths and/or polarizations in order to ensure the ability of the alignment sensor to provide reliable position information. The addition of more patterns may reduce the amount of light scattered by alignment marks on the substrate. Furthermore, some patterns may be made of materials that are opaque to the wavelengths used by the alignment sensor. To mitigate this, even more complexity must be added to the alignment system. However, given the physical constraints on the alignment sensor, since it has to fit within the lithographic apparatus, may not be feasible or desirable.
Furthermore, methods to increase the visibility of alignment marks to the radiation emitted by the alignment system are time consuming. This slows down the production speed of the lithographic apparatus. Additionally, the methods require additional patterns to be applied, each of which must itself be aligned with the substrate.